1. Field of the Disclosure
This disclosure relates to a method of geophysical prospecting which improves the accuracy of seismic migration. Specifically, the disclosure uses a redatuming method followed by vector migration of VSP survey data for imaging of reflections below salt layers in the earth and salt boundaries.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver. Salt layers in the subsurface are particularly problematic. Due to the high compressional wave (P-wave) velocity of salt (4.48 km/s or 14,500 ft/s), there is considerable ray-bending of P-waves at the top and bottom of salt layers due to the large velocity contrast. Typical sedimentary velocities in the Gulf of Mexico may be no more than 3 km/s.
Numerous approaches have been taken to address the problem of sub-salt imaging. These include using low frequencies, use of prestack depth migration, use of converted waves, redatuming to the base salt reflection, and seismic inversion. These have had limited success.
U.S. patent application Ser. No. 11/684,378 of Lou et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, teaches the use of a walkaway Vertical Seismic Profile (WVSP) survey to estimate sub-salt velocities by tomographic inversion of reflection travel-times. In a WVSP survey, measurements are made using a plurality of receivers in a borehole responsive to excitation of one or more seismic sources at a plurality of distances from the wellbore. The estimated velocities may then be used for migration of the walkaway VSP data or of surface seismic data. This method is particularly useful in the drilling of offset wells where an initial well that may or may not be productive has been drilled.
Zhao et al. (2006) disclose the use of an offset VSP to image the flanks of salt structures close to a nearby borehole. The offset VSP has advantages over the conventional refraction salt proximity survey in areas where the surrounding sediment velocity is not available. Such migration methods suffer from two drawbacks. The first is the necessity of determining a complex overburden velocity model. The second is the distortion in the seismic signal that travels through a complex overburden.
Yu et al. (2007) discuss the use of interferometric imaging and application to VSP imaging of salt flanks. The principles of seismic interferometry, also referred to as the virtual source method, is becoming a popular technology in VSP data processing to image complex subsurface structures (e. g. salt flanks) under complicated, and often poorly understood, overburden formation (Bakulin and Calvert 2004 &2006, U.S. Pat. No. 6,747,915 to Calvert, Hornby and Yu 2006, Lu et. al. 2007, Mateeva et. al. 2007, Yu and Hornby 2007). Using virtual source technology, surface sources can be redatumed to the borehole receiver positions, in effect creating a series of virtual common shot gathers for each receiver in the borehole. There are two major advantages in using virtual source technology in VSP data processing. The first is the ability to avoid the determination of a complex overburden velocity model necessary for proper image migration. The second is the removal of distortion in the seismic signal that travels through a complex overburden by positioning a virtual source close to the target zone(s).
The prior art methods using interferometric imaging are limited to single component data and performing a scalar migration of single component data. The present disclosure uses multicomponent data (up to 3-receiver components and up to 3 source components). The limitations of single component data in this context had not been recognized previously. The results are unexpectedly better than would be expected simply from considerations of the sine and cosine relationships between the three components.